Two common forms of storage device on a computer system are the magnetic disc drive and the magnetic tape drive. Computer systems have established methods of handling each of these two types of storage peripherals. Media created on such devices for a given standard are portable to other systems which employ devices which adhere to the same standard.
Recently another type of storage peripheral has become available on many systems; the compact disc device. It is common for such devices to be read-only in nature. A computer system cannot write information to media on such devices but can only read information from media on the device. Compact discs provide the advantage of a high volume of storage in a relatively small package which is highly portable. In addition, the use of such discs permits access to the information in a rapid manner.
An industry standard for file management on CD-ROM disc has been promulgated by the International Standards Organization under the designation ISO 9660. This standard has been adopted by much of the industry and there are numerous systems which read CD-ROMs which adhere to the ISO 9660 standard.
There is a storage device which permits a degree of output capability for optical media. The device is known as a Write Once Read Many (WORM) disc drive and it provides the capability to perform a write to an optical medium. Once written, the medium is not erasable.
There is no information management system standard for writable optical discs. If such discs are managed in the same manner as a magnetic disc file, such as that used on hard disc or floppy disc, there are a number of drawbacks for a WORM device. Specifically, such methods tend to be extremely slow and to waste much of the available medium.
The methods are slow because the file management scheme requires that file allocation tables and directories be maintained on the medium with the data. Each update to the file causes updates to the directories, tables, and the data area. This is not a problem with magnetic medium since it is erasable and does not result in a loss of storage capacity. However, on WORM medium each time the file is updated, the medium's storage capacity is reduced. Multiple recordings of the file allocation tables and directories during a single update can become especially slow since the information must be rewritten each time.
One means to reduce the performance and storage capacity impact of maintaining the necessary tables and directories to manage a WORM medium in the same manner as a magnetic disc is to employ a cache and to maintain a temporary copy of the necessary data structures in the cache. However, this leads to obvious vulnerabilities since a failure of the cache results in loss of the data structures. This risks loss of the data written to the medium due to an inability to locate it.
Another problem with managing WORM media like magnetic disc media is managed is that the storage capacity of the medium constantly decreases as overwrites to the existing files occur. In addition, the ability to overwrite in such files may suddenly disappear. That is, while the length of a file might not change, changes within that file may cause the entire WORM disc space to be exhausted. Eventually, overwrites to the file will no longer be possible when all available space on the disc has been used once. It is not a true erasable disc.
Another problem with this method is that it masks whatever indelibility characteristics the WORM device might otherwise possess. Such indelibility can be valuable in employing the medium as a form of permanent archive suitable for evidentiary purposes. This value is masked because the attempts to tamper with the data after it is written would actually be aided by the system which permits data to be overwritten in a logical fashion. Only an arduous backtracking of the historical sequence of writes to the disc could detect when a malicious or accidental altering of the data was performed, if at all.
Yet another drawback with this approach involves the method chosen to manage files on the medium. Depending on the method, a medium may only be usable in a system which created it.
Another possibility is to manage a WORM device as a tape drive. Like the disc solution above, this method is fraught with problems. For instance, this method hides the random access nature of WORM devices. It also hides the shared use capability of a WORM device. That is the ability for more than one user to access files on the device at the same time. Such methods also often employ the buffering described above which risks the integrity of the data.